The surface stability of superalloys is a significant problem for advanced industrial gas turbines. Highly corrosive environments are generated by the combustion of heavy fuel oils and when coupled with higher firing temperatures and longer maintenance intervals, very strict limitations arise in material selection. The application of an oxidation and hot corrosion resistant sheet cladding alloy to a high strength substrate represents one solution to the surface stability problem.
During the past several years, considerable progress has been made in the development of methods for the diffusion bonding of claddings to the substrates. For example, Schilling et al in U.S. Pat. No. 3,928,901 teach a method in which the sheet cladding is cold isostatically pressed to form a tight skin over the substrate. Beltran et al in U.S. Pat. No. 3,904,101 also teach a process of cladding in which the space between the cladding and the substrate is evacuated, all seams are vacuum brazed, and thereafter the assembly is diffusion bonded in an autoclave using a gaseous medium and elevated temperature and pressure. Schilling et al, U.S. Pat. No. 3,952,939, teach a process in which a preassembled sheet cladding and substrate is masked at all seams, surrounded with glass chips and then hot diffusion bonded while melting the glass and ensure an isostatic stress state. In application Ser. No. 785,303, filed Apr. 6, 1977 now abandoned and of common assignee as the instant application, Schilling et al disclosed a method in which the seams of the cladding are masked, a vacuum to outgas the part and the glass is drawn, and after reestablishing atmospheric pressure by backfilling the vacuum apparatus with nitrogen or argon, hot isostatic pressure is applied using molten glass as a sealant to ensure a pressure differential across the clad/substrate interface.
The cladding processes currently employed involve a relatively large number of steps, some of which are highly labor intensive and are therefore costly. These techniques are also difficult to apply to the more complex substrate configurations such as multivane nozzle segments.
It is the object of this invention to provide a new method for diffusion bonding sheet claddings to complex part configurations which eliminates several of major steps employed in the prior art while simultaneously offering the capacity for cladding difficult substrate configurations. This and other objects of the invention will become apparent to those skilled in the art from the following detailed description taken in connection with the FIGURE which illustrates sequential steps in practicing the present invention.